The present invention relates to an automatic exposure control circuit for use in a photographic camera and, particularly, to such control circuit to be supplied with a film sensitivity information as an electric signal.
An automatic exposure control circuit for controlling an exposure time of a camera has been known which comprises an electromagnet for closing a shutter of the camera, a time constant circuit composed of a light receiving element and a capacitor for commencing an open motion of shutter blades while starting a charging of the capacitor and a driving circuit responsive to a charge voltage of the time constant circuit and a predetermined reference voltage for rendering the electromagnet to close the shutter when the charge voltage reaches the reference voltage. FIG. 1 shows an example of such automatic exposure control circuit, in which, when a camera shutter is started to open by means of a depression of a shutter release button, a trigger switch 1 ganged with the shutter release button is opened to allow a capacitor 3 to be charged through a light receiving element, e.g., CdS element 2. When a voltage of the capacitor 3 reaches a predetermined reference voltage Vt of a driving circuit 4, the latter blocks a current flow through an electromagnet 5 to close the shutter. A shutter opening time T between time instances of the turning off of the trigger switch 1 and of the deenergization of the electromagnet 5 is determined by a time constant of a time constant circuit composed of the CdS element 2 and the capacitor 3. Since a resistance value of the CdS element 2 depends upon an intensity of an incident light from an object to be photographed, the shutter opening time T varies with a variation of the incident light intensity according to the following equation. ##EQU1## where C is a capacitance of the capacitor 3, Ro is a resistance value of the CdS element 2 when the light intensity is Xo, .gamma. is a .gamma. value of the CdS element, x is the intensity of light fallen on the CdS element and corresponds to Ev value at ASA 100, Vcc is a source voltage and Vt is the reference voltage.
The control circuit in FIG. 1 further includes a resister 6 for high intensity correction and a resistor 7 for low intensity correction, a winder 8 for setting the film sensitivity being provided in front of the CdS element 2.
A preciseness of exposure obtained by the automatic exposure control circuit shown in FIG. 1 will be explained with reference to FIG. 2 in which various Ev-T curves and Lv-T curves are shown.
In FIG. 2, the Ev-T curves show relations of the shutter opening time, i.e., the shutter speed T which is the time period between the turning off of the trigger switch 1 and the deenergization of the electromagnet 5 to the exposure value Ev (=diaphram value Av+shutter speed Tv) and are determined by a shutter mechanism . The Lv-T curves show relations of the time period T to a light value Lv (=incident light amount Bv+film sensitivity Sv) and are determined by the equation (1).
The preciseness of exposure or exposure error (dEv) can be represented by a difference between the Ev-T curve and the Lv-T curve.
Assuming that the control time is T when the CdS is irradiated with light whose intensity is Lv, the shutter mechanism provides the exposure amount Ev. If Lv=Ev in this case, the exposure error is 0. However, there is a difference between Lv and Ev, practically, and thus the exposure error dEv (=Lv-Ev) is produced as shown in a lower portion of FIG. 2.
The low intensity correction or the high intensity correction is performed to minimize the error dEv. The term "low intensity correction" used in this specification means a regulation of the error due to differences in the capacitor 3, the resistance Ro of the CdS element and the .gamma. value of the same etc. between cameras by changing the reference voltage Vt by a regulation of the resistance of the resistor 7 so as to minimize the error. That is, it is assumed that a curve A in FIG. 2 is the most preferable curve providing the minimum exposure error. When the characteristics of a certain camera is shown by a curve A' which is deviated from the curve A by the differences in values of the constituting elements thereof from a camera having the characteristics curve A, the value of the resister 7 is reduced so that the curve A' is shifted to the position of the curve A. Since, in this case, the regulation of the curve A' to the curve A is performed as a whole, it may be enough to shift any one point on the curve A' to a corresponding point on the curve A. Since such shift is usually performed for a point corresponding to around Lv 9, it is referred to as the low intensity correction.
On the other hand, the high intensity correnction intends to make the Lv-T curve close to the Ev-T curve. That is, the Ev-T curve is constituted with two segments, one being in a low intensity side which is stable for any camera and close to a corresponding portion of the Lv-T curve and the other being in a high intensity side which depends upon cameras and is substantially different from a corresponding portion of the Lv-T curve. A junction of the two segments corresponds substantially to a position in which the shutter blade is fully opened.
In order to minimize the error dEv it is necessary to make the Lv-T curve, particularly, the high intensity side portion thereof, as close to the corresponding portion of the Ev-T curve as possible. This can be achieved practically by regulating the resistance value of the resister 6 connected in series to the CdS element 2 of the circuit shown in FIG. 1.
Assuming the resistance value of the resister 6 as being r, the control time T can be represented according to the equation (1), as follow. ##EQU2## Therefore, it is possible to make the Lv-T curve overlapped with the Ev-T curve by regulating the value of only the term C r ln Vcc/(Vcc-Vt) by changing the resistance value r of the resister 6. Since the portion of the Lv-T curve which is changed as above corresponds to the light value (Lv) of around 15, this regulation is referred to as the high intensity correction.
In this manner, the variations of the circuit constants such as the capacitance of the capacitor and the .gamma. value of the CdS element etc. of the control circuit and/or mechanical variations of camera are corrected.
On the other hand, an information of the film sensitivity which is one of parameters necessary to operate the control circuit appropriately is given as a change in a diameter of the window 8 formed in front of the CdS element 2 and is supplied, together with the amount of the incident light, to the control circuit as Lv=Bv+Sv. Such input of the film sensitivity information through the light receiving window 8 for the CdS element to the control circuit can be represented by the Lv-T curve shifted horizontally.
In a case where ASA100 is selected as a reference, for example, a case of ASA400 can be represented by the Lv-T curve shifted leftwardly by 2Ev, as shown in FIG. 4.
In any way, however, the film sensitivity information has to be input to the control circuit manually, i.e., by changing the size of the opening of the window 8 manually according to the sensitivity of a film loaded.
In order to make an automatic input of the film sensitivity information to the control circuit possible, an idea has been proposed that an electric contact coded according to the film sensitivities is provided preliminarily on a patrone of a film to be loaded and, when loaded, a code signal is derived from the electric contact, on which the sensitivity of the loaded film is detected and supplied to the control circuit automatically according to which various parameters of the circuit are changed.
This proposition seems to be very effective. However, any construction for practicizing this idea has not been proposed as yet.
That is, a construction of the control circuit for practicizing the idea may be one shown in FIG. 3, in which a coded film sensitivity signal derived from the contact on the patrone of a certain film loaded is supplied to a decoder 9. An output of the decoder 9 which corresponds to one of the film sensitivities is supplied to a corresponding one of switching transistors 10 to turn it on to thereby connect a corresponding one of resisters 11 to a reference voltage input of the driving circuit 4. Thus, a reference voltage suitable for the sensitivity of the loaded film can be established at the reference voltage input of the driving circuit 4. The switching of the reference voltage one to another in response to the film sensitivity can be represented by a vertical shaft of the Lv-T curve, as mentioned previously.
However, a mere vertical shift of the Lv-T curve causes various problems to occur, which are as follows:
Firstly, when the curve is shifted vertically by a difference dSv in the film sensitivity with a reference being set in the low intensity side, there is provided in the high intensity side a difference which in larger than the value dSv, causing an exposure error, as shown in FIG. 5. That is, when the amount of shift in the low intensity side is 2Ev, the amount of shift in the high intensity side which should be 2Ev as shown by a curve C is much larger than 2Ev.
Secondly, because of a variation of the .gamma. value of the CdS element, the larger the difference between the sensitivity of a film to be loaded and the reference film sensitivity provides the larger the exposure error, as shown in FIG. 6. In FIG. 6, a curve A shows the Ev-T curve of a camera mechanism as well as the Lv-T curve adaptable thereto, where the .gamma. value of the CdS element is .gamma.o and the film sensitivity is ASA100, and a curve A' shows those for ASA400. In this case, if there is no variation of the .gamma. value of the CdS element, the curves A and A' are parallel. However, there is a variation of the CdS element camera by camera, practically.
Assuming that the .gamma. value of a certain CdS element is not .gamma.o but .gamma.' and a curve B is obtained as shown in FIG. 6, a regulation is performed at a shutter speed To at which the curves A and B crosses. The exposure error at a certain shutter speed is e.
When the switching of the film sensitivity to ASA400 is performed by changing the window size of the CdS element as in the conventional manner, the curve B is merely parallel-shifted resulting in a curve C. Therefore, the exposure error at the certain shutter speed is .DELTA.e.
On the other hand, when the switching is performed by changing the reference voltage Vt of the driving circuit as mentioned, the amount of parallel-shift is increased, resulting in a curve B' as shown in FIG. 6. Thus, an additional exposure error .DELTA.E is included, which increases with an increase of the difference in film sensitivity.
Finally, there must be a low intensity correction means provided additionally since the switching of the reference voltage Vt which is used for the low intensity correction in the conventional system is used as the film sensitivity switching.